This invention relates to an electrochemical immersion sensor for the determination of the concentration of a metallic component in a molten metal, comprising the molten metal as the measuring electrode and a reference electrode, the latter containing the metallic component to be measured, separated from each other by a liquid ion-conducting halide containing the metallic component to be measured and immobilised in a non-conducting porous support fabricated from a material which is inert or almost inert to the molten metal, the halide and the reference electrode material, wherein the sealing of the reference electrode is at least partly provided by the molten metal itself and wherein the reference electrode is introduced by a melt process. This invention further relates to a method for producing said sensor. The term xe2x80x9cmolten metalxe2x80x9d as used herein is understood to mean the melt of a metal or alloy.
An electrochemical sensor, in particular able to measure the aluminium concentration in a molten metal, is known from patent application EP 0 493 878 A2. The sensor comprises a gas tight holder fabricated of quartz or pyrex with a projection attached to the tip, which is removable by snapping in use to allow the enclosed ion-conducting material or electrolyte to contact the molten metal. NaClxe2x80x94AlCl3 electrolyte is used as the ion-conducting material whereby the NaCl acts as saturated solid component. A pure aluminium wire immersed in the ion-conducting material is used as the reference electrode, whereas the molten metal itself serves as the measuring electrode. In a particular embodiment of the invention, a dense xcex2-alumina membrane immersed in the ion-conducting material separates the reference and measuring electrodes. If the composition of the electrolyte remains constant, the aluminium activity at the reference electrode is fixed and known, and if the aluminium activity at the measuring electrode in the molten metal is established, the sodium activity at both sides of the membrane will be known and the value of the aluminium activity at the measuring side can be determined by the following equilibrium: 3 Na+AlCl3=3 NaCl+Al. A sodium concentration cell is obtained. By measuring the EMF of this sodium concentration cell, the aluminium activity or concentration in the molten metal can be deduced from Nernst""s equation.
Most important disadvantage of said sensor is its fragility (the salt can easily be lost), due to which said sensor can not be used in agitated molten metals. Another disadvantage of said sensor is the stringent requirements to the composition of the electrolyte. Another disadvantage is the indirect determination of the aluminium concentration via a sodium concentration cell. It""s further disadvantageous that said sensor can not be used in liquid aluminium since most of the aluminium alloys contain sodium disturbing the above mentioned sodium equilibrium. User practice has further indicated that said sensor should be sufficiently immersed in the molten metal (at least 20 cm).
Other publications are xe2x80x9cImmobilised Molten Salt Membrane based Magnesium Sensor for Aluminium-Magnesium Meltsxe2x80x9d, Vangrunderbeek et al., Ionics 1 (1995) p. 59-62, and xe2x80x9cElectrochemical Sensor for Measuring Magnesium Content in Molten Aluminiumxe2x80x9d, Zhang et al., Journal of Applied Electrochemistry, 26 (1996), 269-275. These documents describe sensors for measuring the Mg activity in Alxe2x80x94Mg melts. Disadvantage of said sensors is the insufficient sealing of said sensors with cements for use in an industrial process.
This invention is aimed to provide a new electrochemical sensor for continuously measuring of the concentration of a metallic component in a molten metal in an industrial environment. Another aim of this invention is to provide a method to produce such a sensor.
The first embodiment of this invention is an electrochemical sensor to measure the activity of a metallic component in a molten metal, comprising the molten metal as the measuring electrode and a reference electrode, the latter comprising the metallic component to be measured, separated from each other by a liquid ion-conducting halide comprising the metallic component to be measured and immobilized in a non-conducting porous support fabricated from a material substantially inert to the molten metal, the halide and the reference electrode material, and whereby the reference electrode further comprises an external connection comprising electrically conducting wire immersed in an electric isolating material which is chemically substantially inert to the molten metal and the reference electrode material, characterized in that the sealing of the reference electrode is provided by a high temperature cement and the molten metal itself and by a gas tight sealing of the external connection above the melt, and by melting the reference material inside the electrochemical sensor. The porous support is preferably shaped as one closed end tube.
The liquid ion-conducting halide preferably contains chlorides, fluorides and/or bromides, of which at least one comprises the metallic component to be measured.
The porous support preferably has porosity between 20 and 50%, most likely between 30 and 40%. As porosity is higher the strength of the porous support will be lower, leading to a limited applicability in an industrial process. When porosity is too low, the conductivity of the impregnated halide will be negatively influenced resulting in an increase of the reaction time of the sensor upon immersion in the molten metal and in a decrease of the obtained accuracy. When the porosity and the pore size are measured by a Coulter Porometer(copyright) and Coulter Porofil(trademark) wetting agent, a minimal test gas flow (pressurized air) of 50% should be measured for the pores between 0.5 and 5 xcexcm, most likely between 0.5 and 1.5 xcexcm. The pores are open pores permitting ionic transport. When the average pore size is lower, the ion conductivity of the halide is too low to obtain useful measurements. When the average pore size is higher, the impregnated halide can leave the porous support more easily and the molten metal can penetrated the porous support, making the sensor unclear.
In another embodiment, the porous support is manufactured of MgO. The MgO powder used to fabricate the porous support preferably has a purity of at least 99.5%.
In another embodiment of this invention, the porous support has been obtained by compacting and subsequent sintering of a MgO powder with a grain size distribution of at least 200 mesh or a largest grain size of 74 micrometer.
For the determination of Mg in an a melt of an Al alloy, the ceramic tube used to protect the electrically conducting wire for the external connection of the reference electrode, is preferably not made of SiO2 containing material, since SiO2 reacts with the Mg used in the reference. The electric contact with the reference electrode is obtained by connecting the pure Mg with a suitable electrically conducting wire, preferably Mo, Ta or W. The electric contact with the measuring electrode can be very easily obtained via an electrically conducting wire in the molten metal, preferably made of the same material as the electrically conducting wire of the reference electrode.
In a further embodiment, the above-described embodiment is contained in a holder made of a material, which is substantially insoluble in the molten metal.
Said holder can be characterized in that it is provided with a ceramic or refractory material in the vicinity of the metal surface. In a preferred embodiment, said holder is made of a functional conducting material so that said holder serves at the same time as an electric connection for the measuring electrode of the electrochemical sensor. The holder can contain a thermocouple as well.
A second main embodiment of this invention is a method to fabricate an electrochemical sensor to measure the activity of a metallic component in a molten metal, comprising the melt as the measuring electrode, a reference electrode, the latter comprising the metallic component to be measured, separated from each other by a liquid ion-conducting halide comprising the metallic component to be measured and immobilized in a non-conducting porous support fabricated from a material substantially inert to the molten metal, the halide and the reference electrode material and wherein the reference electrode contains an external connection consisting of an electrically conducting wire in an electric isolating protection material, chemically substantially inert, characterised in that the method is built up according to the following sequential steps:
sealing of the porous support containing the reference electrode material and the external connection, using a high temperature cement,
immobilizing the halide into the porous support at a temperature above the melting temperature of the reference electrode material or melting of the electrode material followed by immobilizing the halide into the porous support at a temperature lower than the melting temperature of the reference electrode material so that in both cases the reference electrode material is introduced by melting the reference material inside the electrochemical sensor,
sealing of the external connection of the reference electrode above the melt using a gas tight paste, and
in-situ completion of the sealing of the sensor by totally immersing the porous support of the sensor under the melt surface.
A further characteristic of this invention is the use of the sensor as described above or manufactured according to the method as described above for measuring the activity of a metallic component in a molten metal.
The electrochemical sensor according to this invention is in particular able to continuously measure during several hours the concentration of a metallic component in a molten metal. For example, the sensor can be immersed in a molten metal in the runner as well as in the foundry furnace or any other melt, even inductively heated. Due to the short reaction time upon immersion, the sensor also can be used for single shot measurements, which last only for a couple of minutes.
By selecting suitable components, the sensor of this invention can be introduced for continuously measuring of the concentration of different metallic components in a variety of molten metals.
This invention will now be described by a number of examples and drawings that are non-limiting to the scope of this invention.